The present invention relates to digital imaging and, more particularly, to a technique for identifying defective pixel regions in a digital image detector, such as detectors used in digital X-ray imaging systems.
Discrete pixel imaging systems, such as digital X-ray imaging systems, typically employ a detector for encoding information within specific picture element regions or pixels. In a digital X-ray system, for example, portions of a radiation stream emitted by an X-ray source penetrate a subject of interest and impact a detector placed adjacent to the subject. Based upon the structure and composition of the subject, varying amounts of radiation will impact the detector at the pixel locations. The detector is configured to generate output signals for each pixel location which are representative of the amount of radiation impacting the particular discrete regions defined by the pixels. These signals are then transmitted to processing circuitry for further conditioning, filtering and enhancement, eventually to be reconstituted into an overall image for use by an attending physician or radiologist.
In discrete pixel imaging detectors, such as those used in digital X-ray systems, it is not uncommon for detector output levels to vary between pixels, even when the pixels are exposed to equal levels of radiation. Such variations may be due to tolerances within the sensitivity of the detector itself, as well as to various forms of noise which may occur in the detection system. However, while certain normal variations may be permitted, significant differences in pixel-to-pixel output from the detector are not desirable.
Such pixel-to-pixel output variations may involve both underactive pixels (i.e., those regions producing a signal significantly lower than other regions for the same received radiation) and overactive pixels (i.e., regions producing output levels significantly higher than other regions for the same received radiation). In addition to producing erroneous dark or light artifacts in the resulting image, data from such underactive or overactive pixels can adversely affect signal processing operations performed on the image, such as adjustment of contrast and tone, as well as errors in dynamic range detection and image enhancement.
Defects in pixels in solid state detectors may result from various causes. For example, high leakage currents, open circuits and short circuits can cause pixels erroneously to output signals when no significant radiation levels have impacted their locations, or to output abnormally low signal levels when radiation has impacted the pixels. It would be useful, therefore, to identify potentially defective detector pixels so as to avoid erroneous data in discrete pixel images produced from the detector output. Where significant output differences are detected between pixels of a detector, it may be useful to flag such pixels as defective, and to manage information they provide in a special manner, or simply to disregard their output.
A technique is provided for identifying defective pixels in a discrete pixel detector, such as detectors used in X-ray imaging systems. In an exemplary embodiment of the invention, the technique subjects the detector to both dark and light image tests wherein a series of readings are obtained both in the absence of X-ray exposure and in the presence of uniform exposure over the matrix of pixels comprising the discrete pixel image. In the dark image sequence, noisy or overactive pixels are identified based upon statistical comparisons of the pixel output data. Pixels producing intensity values that fall outside a specified range are flagged as potentially defective. Underactive pixels are also identified in the dark image sequence. In the exposed image sequence, again both overactive and underactive pixels are identified which fall outside a desired deviation band. Moreover, pixels may be flagged as potentially defective if their output intensity value varies by more than a desired amount from an average value. Even where so identified, the pixels may be found to be acceptable through a local neighborhood comparison with values of pixels closely adjacent to the potentially defective pixel.
The technique provides an effective and efficient procedure for automatically identifying potentially defective pixels in detectors. The technique is particularly well-suited to quality control testing, as well as to calibration or installation verification tests of detectors in newly-installed and existing imaging systems. Based upon the results of the identification procedure, output signals from pixels identified as defective may be conditioned or disregarded by image processing circuitry to reduce or compensate for their presence in the detector. Moreover, the technique can be used to identify detectors which are acceptable for service, as well as detectors having unacceptable numbers of defective pixels, and which therefore should not be placed in service or should be removed from service.